In order to gain insight into the determinants of nonpathogenic SIV infection in the naturally infected sooty mangabey host, SIV replication in mangabeys was assessed and compared to SIV replication observed in pathogenically infected rhesus macaques. Application of an SIV quantitative competitive PCR assay developed in the lab demonstrated that high level viremia in non-pathogenic SIV infection of mangabeys represents high levels of viral replication and cell turnover. In situ methods were developed to define the phenotypes, numbers, and anatomic distribution of SIV-infected cells in monkey tissues that are responsible for this high level viremia. In situ hybridization (ISH) for SIV RNA using non-isotopic SIVmac probes was applied to detect SIV RNA in lymphoid tissues of mangabeys and macaques. Immunohistochemical procedures for the identification of macrophages, T cells and dendritic cells in fixed tissues were also implemented. Combination IHC with ISH demonstrates the SIV-infected cells in both species to be macrophages and T lymphocytes. Preliminary comparisons suggest that the number of infected cells in mangabeys may be less than that observed in macaques with similar levels of plasma viremia. In order to determine whether pa thogenic SIV infection differs from nonpathogenic SIV infection in terms of the host response to virus, an in situ apoptosis detection method was implemented. The number of apoptotic cells in SIV-infected mangabey tissues appears comparable to that observed in infected macaques, suggesting that the lack of CD4 cell depletion in mangabeys is not due to decreased apoptosis in response to SIV infection. Confirmatory studies to extend these observations are in progress. A major focus in the coming year will be completion of the implementation of a kinetic PCR method for the high throughput quantitation of SIV nucleic acid in infected monkeys.